1.{) 0 0 C\.1 ..!. 0.. <C ~ U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY GEOLOGIC MAP OF UPPER EOCENE TO HOLOCENE VOLCANIC AND RELATED ROCKS IN THE CASCADE RANGE, WASHINGTON By James G. Smith ....... (j, MISCELLANEOUS INVESTIGATIONS SERIES 0 0 Published by the U.S. Geological Survey, 1993 ·a 0 0 3: )> i:l T t'V 0 0 (J1 U.S. DEPARTMENT OF THE INTERIOR TO ACCOMPANY MAP 1-2005 U.S. GEOLOGICAL SURVEY GEOLOGIC MAP OF UPPER EOCENE TO HOLOCENE VOLCANIC AND RELATED ROCKS IN THE CASCADE RANGE, WASHINGTON By James G. Smith INTRODUCTION the range's crest. In addition, age control was scant and limited chiefly to fossil flora. In the last 20 years, access has greatly Since 1979 the Geothermal Research Program of the U.S. improved via well-developed networks· of logging roads, and Geological Survey has carried out a multidisciplinary research radiometric geochronology-mostly potassium-argon (K-Ar) effort in the Cascade Range. The goal of this research is to data-has gradually solved some major problems concerning understand the geology, tectonics, and hydrology of the timing of volcanism and age of mapped units. Nevertheless, Cascades in order to characterize and quantify geothermal prior to 1980, large parts of the Cascade Range remained resource potential. A major goal of the program is compilation unmapped by modern studies. of a comprehensive geologic map of the entire Cascade Range Geologic knowledge of the Cascade Range has grown rapidly that incorporates modern field studies and that has a unified in the last few years. Luedke and Smith (1981, 1982) estimated and internally consistent explanation. that, when their maps were made, more than 60 percent of the This map is one of a series presently being compiled that Cascade Range lacked adequate geologic, geochemical, or shows Cascade Range geology by fitting published and unpub­ geochronologic data for a reliable map at 1:1,000,000 scale. lished mapping into a province-wide scheme of lithostrati­ Today only about 20 percent of the range in Washington lacks graphic units; map sheets of the Cascade Range in California adequate data to be shown reliably at the larger 1:500,000 scale and in Oregon complete the series. The complete series forms a of this map. Areas that remain poorly understood in Washington guide to. exploration and evaluation of the geothermal resources include the stratovolcanoes, Mount Baker and Mount Rainier, of the Cascade Range and will be useful for studies of volcanic and Eocene to Miocene rocks of the western part of the hazards, volcanology, and tectonics. Cascade Range between lats 122°30' and 123°. For geothermal reasons, the maps emphasize Quaternary This present series of maps of the Cascade Range is not volcanic rocks. Large igneous-related geothermal systems that merely a reworking of previously published data. Geologic have high temperatures are associated with Quaternary volcanic interpretations shown here are based largely on newly published fields, and geothermal potential declines rapidly as age increases and unpublished geologic maps and radiometric determinations, (Smith and Shaw, 1975). Most high-grade recoverable geo­ including my own, done since 1980. To assign all map units their thermal energy is likely to be associated with silicic volcanism correct composition and age, I also reevaluated older published less than 1 Ma. Lower grade(= lower temperature) geothermal maps and incorporated recently determined chemical analyses resources may be associated with somewhat older rocks; and radiometric ages. however, volcanic rocks older than about 2 Ma are unlikely geothermal targets (Smith and Shaw, 1975). DISCUSSION· Rocks older than a few million years are included on the map INTRODUCTION because they help to unravel geologic puzzles of the present­ The Tertiary and Quaternary volcanogenic Cascade Range day Cascade Range. The deeply eroded older volcanoes found in Washington is divided into two segments by a northeast­ in the Western Cascades physiographic subprovince are trending line (fig. 1; hereafter referred to as the northern analogues of today's snow-covered shield volcanoes and Washington and southern Washington segments). Weaver and stratovolcanoes. The fossil hydrothermal systems of the Eocene Michaelson (1985) originally drew this line parallel to the to Pliocene vents now exposed provide clues to processes present-day direction of plate convergence between the Juan active today beneath the Pleistocene and Holocene volcanic de Fuca and North American plates, basing its location on the peaks along the present-day crest of the Cascade Range. Study pattern of present-day seismicity in Washington and northern of these older rocks can aid in developing models of geothermal Oregon. However, the two segments differ in several important systems. These rocks also give insight into the origins of geologic aspects, such as rate of volcanic production, abundance volcanic-hosted mineral deposits and even to future volcanic ofTertiary plutonic rocks, and style of deformation, suggesting hazards. that the line has a significance beyond the location of earthquake Historically, the regional geology of the Cascade Range has hypocenters. been interpreted through reconnaissance studies of large areas Differences in geologic characteristics of the segments that (for example, Diller, 1898; Williams, 1916; Callaghan and typify the present-day Cascade Range have existed at least Buddington, 1938; Williams, 1942, 1957; Peckand others, 1964; since Eocene time. For example, during the last million years, Hammond, 1980). Early studies were hampered 'by limited the southern Washington segment produced nearly 7 times access, generally poor exposures, and thick forest cover, which more intermediate-composition and silicic magma per kilometer flourishes in the 100 to 250 em of annual precipitation west of ofarc length than the northern Washington segment. From late 1 North American Pacific plate plate I l<tr ;_,?:>~ ? ~ Axial 4. seamo ..,.,__0 cf v PACIFIC I (tyc::- , 1 ~~ OCEAN OREG ------- / Juan /~lvc de Fuca ~ ·L.b~~-'?c pI ate '- ~ -ru. ', I ~~<i ', OJv~ 124° 123° 60 KILOMETERS Figure 1. General setting of late Eocene to present-day Cascade Range in Washington showing selected geologic features. EXPLANATION well known in either segment. These persistent differences must be taken into account in any reconstructions of the Pacific Holocene to Pliocene stratovolcano--Star indicates Northwest. In the following section the characteristics of each peale MB, Mount Baker; GP, Glacier Peak; MR, segment are discussed in greater detail. Mount Rainer; GR, Goat Rocks volcano; MA, Mount Adams; MSH, Mount St. Helens NORTHERN WASHINGTON SEGMENT Volcanism Major Holocene to Pliocene basalt field Quaternary volcanogenic rocks are not abundant in this segment of the Cascade Range, and volcanic production is low compared to other parts of the Cascade Range (Sherrod, 1986; Pliocene to late Eocene volcanogenic rocks of the Sherrod and Smith, 1989b). Volcanic activity in this segment is D Cascade arc found at the active stratovolcanoes Mount Baker and Glacier Peak and at minor basalt flows and cinder cones 10 to 20 km Middle Miocene to late Eocene granitic epizonal intru­ south of Glacier Peak. The Mount Garibaldi volcanic field, 150 sive rocks km northwest of Mount Baker in British Columbia, is also included in this segment because it is similar to Mount Baker -- Line dividing northern Washington segment of the and Glacier Peak. Each of these areas has active stratovolcanoes Cascade Range from southern Washington seg­ at its center that have erupted mainly intermediate-composition ment-Based on earthquake hypocenters; from Weaver and Baker ( 1988) and silicic lava. 40--- Depth contour of Juan de Fuca plate underneath conti­ As with other estimates that follow, only an approximation of nent-In kilometers; from Weaver and Baker the volume of volcanic products per unit time is possible for this (1988) segment. Glaciers and streams have removed much of the Deformational front at base of continental slope­ evidence; older valley-filling pyroclastic and debris-flow deposits Sawteeth on upper plate. Interpreted as surface ex­ are especially likely to have been eroded. Nonetheless, an pression of subduction zone marking boundary estimate of volcanic production expressed in units of cubic between Juan de Fuca and North American plates; kilometers per kilometer of arc length per million years [km3 from Snavely (1987) and Drummond (1981) (km of arc length)-1 m.y.-1] enables comparison between Location of active spreading-ridge segments-Queried segments, as long as its approximate nature is kept in mind. where uncertain; from Morton and others (1987) Estimates of the volume of magma produced at different along Juan de Fuca ridge and Embley and others volcanic centers are shown in table 1. (1987) along Blanco fracture zone Mount Baker consists predominantly of andesite flows and Active transform faults along plate boundaries-Ar­ subordinate dacite and basalt. The present-day volcano of late rows indicate direction of relative movement; from Pleistocene and Holocene(?) age sits on the eroded remnants of Embley and others ( 1987) one or more middle Pleistocene to possibly late Pleistocene Extensions of transform faults beyond spreading volcanoes, but details of their history are not well known. Rocks ridges-From Embley and others ( 1987) of the present cone as well as older eroded cones are all Relative plate motion-Indicates direction of present­ normally polarized (Swan, 1980). Potassium-argon